OFDM air interfaces will become increasingly important e.g. for future evolutions of air interfaces in 3GPP Radio Access Networks, for Wireless Local Area Networks (WLANs) e.g. according to the IEEE 802.11a standard or for a 4th generation air interface.
In OFDM transmission, time-frequency patterns are allocated to the mobile terminals. Different cells can have different time-frequency patterns, so that no interference coordination between the cells is necessary or possible.
Given the licensed bandwidth, transmission capacity from network providers e.g. for WEB surfing or video streaming has to be as high as possible for all users to serve as many subscribers as possible. Further the quality of service experienced by the user and the coverage of the service is an important property demanded by the user. So OFDM shall also work at the cell border.
A frequency re-use factor of 1 for the different cells shall be achieved for OFDM transmission in order to increase the utilization of the bandwidth without degradation of the quality of service caused by inter-cell interference.
In cellular systems with a frequency re-use factor of 1 the signal to interference ratio at the cell border approaches the factor 1 or 0 dB, so that no useful transmission from the base station to the mobile terminal can be kept up. Therefore in CDMA systems (CDMA=Code Division Multiple Access) soft handover was introduced using a different code from the neighboring cell in addition to the primary code from the serving cell. Additionally both base stations use a differently scrambled pilot signal. For packet transmission using High Speed Downlink Packet Access (HSDPA) no such solution is given reducing the coverage of HSDPA transmission to a fraction of the cell area.
In OFDM transmission, time-frequency patterns are allocated to a mobile terminal instead of codes in CDMA systems. In OFDM transmission, in contrast to CDMA transmission, interference can be planned and avoided. For OFDM transmission, which does not provide different scrambling codes for the different base stations, the problem at the cell border has to be solved as well. For that purpose time-frequency or frequency patterns are allocated to the users and the caused cross-cell interference can be coordinated.
For the coordination of interferences, the frequency patterns have to be the same in neighbor cells while the pilot subgrid in neighbor cells shall be different to allow channel estimation.
Generally to enable channel estimation in the interference region of two cells, it is assumed that pilots and signaling symbols have higher energy than the data symbols, which allows channel estimation even if the pilot of the serving cell falls on the same subcarrier frequency as the data symbol of the interfering cell.
But as a consequence it is necessary that the pilots of the interfering cells do not fall on the same subcarrier frequency. So it is necessary, that the pilot subgrids in neighbor cells shall be different.
The object of the invention is to propose a method for OFDM data transmission in a single-frequency multi-cell mobile network with frequency re-use and channel estimation by means of pilots of a pilot subgrid and without the need for a cell planning for the pilots.